1. Field of the Invention
The present invention relates to a method of preparing lithium hexafluoro phosphate (LiPF6) at a high yield and purity, which is one of the essential elements for a lithium ion secondary battery cell and a lithium polymer battery cell.
2. Description of the Prior Art
A lithium ion battery cell or a lithium polymer battery cell is fundamentally composed of an anode of lithium oxide, such as LiCoO2, LiNiO2, LiMn2O4 or the like, a cathode of a carbon material, such as graphite, and an electrolytic solution consisting of an electrolyte dissolved in an organic solvent. Examples of the organic solvent include nonaqueous solvents, such as a mixed solvent of ethylene carbonate (hereinafter, called xe2x80x9cECxe2x80x9d) and dimethyl carbonate (hereinafter, called xe2x80x9cDMCxe2x80x9d), a mixed solvent of EC and diethyl carbonate (hereinafter, called xe2x80x9cDECxe2x80x9d), and the like. Moreover, lithium hexafluoro phosphate is typically used as the electrolyte.
When the battery cell is electrically charged, Li ion is inserted into, and attached to a void in the cathode, whereas, when it is electrically discharged, the Li ion attached to the cathode is returned to the anode. In this case, the Li ion moves through the electrolytic solution. As a result, in order to maintain performance of the battery throughout its life cycle, lithium hexafluoro phosphate, which is an electrolyte used in the electrolytic solution, is very strictly limited in specifications, such as purity, etc.
As a rule, lithium hexafluoro phosphate is prepared by reacting, in a concentrated hydrofluoric acid solution, lithium fluoride (LiF) with phosphorus pentafluoride (PF5) which is obtained by a reaction of phosphorus pentachloride (PCl5) and hydrofluoric acid (HF). In this reaction, if the reaction system for preparing lithium hexafluoro phosphate contains moisture, lithium oxyfluoro phosphate (LiPOXFY) is produced as a by-product, and lithium oxyfluoro phosphate is also partially resolved into LiF which then remains as an impurity in lithium hexafluoro phosphate. Consequently, it is preferred that the moisture content in the reaction system for preparing lithium hexafluoro phosphate is as low as possible.
A metal component contained in a raw material used in a preparing process of lithium hexafluoro phosphate, or a metal component introduced by a corrosion of an equipment, changes lithium metal ionization potential. Thus, where the metal component is contained in the electrolyte, the life of the battery cell may be shortened.
Furthermore, if moisture is contained in the final product, then lithium hexafluoro phosphate is resolved into LiF, HF, and PF5, that are then converted into a gaseous state, thereby forming an internal pressure with in the battery cell. Also, as HF is reacted with the organic solvent, it has an effect on the corrosion of a case enclosing the battery cell. It adversely affects a stability of the battery cell. For these reasons, lithium hexafluoro phosphate (LiPF6) used as the electrolyte is strictly limited in specifications, such as purity, moisture content, metal content, free hydrofluoric acid content, and the like.
The present invention is a method wherein HF dried completely by means of F2 is used as one of raw materials for the preparation of lithium hexafluoro phosphate to fundamentally prevent lithium oxyfluoro phosphate (LiPOXFY) from being produced as a by-product and also to prevent lithium hexafluoro phosphate (LiPF6) from resolving by moisture, thereby enabling lithium hexafluoro phosphate (LiPF6) to be prepared at a high purity without reducing yield.
Also, the present invention is a method wherein F2 gas of a high purity is introduced at a desired amount during a reaction for preparing lithium hexafluoro phosphate (LiPF6) to fundamentally prevent lithium oxyfluoro phosphate (LiPOXFY) from being produced as a by-product because of moisture contained in a solid type raw material such as PCl5, LiX (X=F, Br, Cl, or I), or the like and other moisture which can be introduced during the reaction, thereby enabling lithium hexafluoro phosphate (LiPF6) to be prepared at a high purity.
In a general method for preparing hydrogen fluoride, the preparation of hydrogen fluoride is carried out after basic raw materials are completely dried to control moisture. In a last step, hydrogen fluoride is passed through anhydrous sulfuric acid to remove moisture contained therein. After undergoing this drying process, hydrogen fluoride typically contains moisture of about 100 ppm, and also sulfuric acid of about 50 ppm. It is a high boiling point material, due to the passage through anhydrous sulfuric acid.
As described above, in the battery cell-grade electrolyte, the content of the metal component is very critical. Because all equipment used for the preparation of HF is made of steel, however, hydrogen fluoride itself will contain a large number of metal components.
Anhydrous hydrogen fluoride resulting from the process is unsuitable for use as a raw material for the preparation of a battery cell-grade electrolyte, because it is high in content of moisture, sulfuric acid, metal components, and the like. Therefore, this needs to be dried and purified, again.
In the present invention, the reaction formula, according to which the remaining moisture in anhydrous hydrogen fluoride is dried, is as follows:
H2O(in HF)+2F2xe2x86x922HF+OF2(↑)
This drying is carried out under the following conditions:
Bubbling time: 1 to 3 hours
Temperature: xe2x88x9210 to 30xc2x0 C.
Flow rate: 1 to 1,000 g/hr
As shown in the reaction formula, as moisture in hydrogen fluoride is reacted with F2 gas to be converted into the hydrogen fluoride, oxyfluoride (OF2) is discharged which is very low in boiling point, such as xe2x88x92145xc2x0 C., so that it is easily volatilized out after the reaction.
The present invention also relates to a purification of PCl5 and LiCl that are solid raw materials. Commercially available compounds generally have respective specifications inherent in purity, moisture content, metal component, and the like. Even though it is well known that the use of a raw material of a high purity results in an increase in quality of a final product, this causes the manufacturing-cost to be increased, thereby decreasing competitiveness. Thus, the present invention adopts PCl5 and LiCl that are relatively cheap raw materials containing some impurities, such as moisture, etc.
As described above, if the raw material contains moisture, lithium oxyfluoro phosphate (LiPOXFY) may be produced which is very difficult to be isolated by a recrystallization.
The present invention is characterized in that F2 gas is introduced into a reaction system to completely remove moisture contained in the solid raw materials, thereby fundamentally preventing lithium oxyfluoro phosphate (LiPOXFY) from being produced.
Moreover, even when lithium oxyfluoro phosphate (LiPOXFY) is produced during the reaction, this is converted into lithium hexafluoro phosphate (LiPF6) and oxyfluoride (OF2) by F2 gas in accordance with the present invention, thereby increasing yield.
In addition, another characteristic of the present invention is that, for controlling the metal components, equipment coated with a fluorine resin, such as polytetrafluoroethylene (hereinafter, called xe2x80x9cPTFExe2x80x9d), is used for all processes, and that pipe lines made of a PFA resin, such as tetrafluoroethylene perfluoroalkylvinylether copolymer are also used. Futhermore, two reactors are used.
The reasons why all the equipment is coated with the fluorine resin and the pipe lines are made of the PFA resin, are as follows.
In the prior art, there was used a reactor made of a material capable of withstanding anhydrous hydrofluoric acid, such as a monel metal, a Ni base alloy, a Cr base alloy, or the like. However, such a material is very expensive, and therefore disadvantageous in view of a commercial factors. Moreover, if this material is exposed to air, then it is in contact with moisture, and it can be corroded.
Where the equipment used in the reaction is corroded, a product will be inevitably contaminated with the corroded materials. Such contaminants act as a factor which increases a metal content in the product.
Where the equipment is contaminated, it must be repaired throughout all processes. This causes a great increase in maintenance and repair expenses, and therefore, results in a process inefficiency.
Furthermore, the main reason why two reactors are used, is because it allows manufacturing-cost to be reduced through the use of a relatively cheap raw material, and also allows the content of the metal components to be decreased.
In lithium hexafluoro phosphate (LiPF6) in accordance with the present invention, either PF5 or PCl5 can be used as a basic raw material. However, it is preferred to use PCl5, because PF5 is expensive compared to PCl5. When HF treated and dried with F2 gas is added to a solid state PCl5 in a first reactor, HF and PCl5 are immediately converted into gaseous PF5 and HCl (a first step reaction).
Therefore, impurities that have been contained in the solid state PCl5 remain in the first reactor, and only PF5 and HCL, each having a high purity, are introduced into a second reactor. As a result, it is possible to prepare a highly pure lithium hexafluoro phosphate (LiPF6) while using a relatively cheap PCl5.
In the second reactor, the following second and third step reactions are carried out in accordance with the present invention.
Reaction formulas and conditions for preparing lithium hexafluoro phosphate (LiPF6) in accordance with the present invention are as follows:
1. Reaction formulas
a) The first step reaction: PCl5+5HFxe2x86x92PF5+5HCl
b) The second step reaction: PF5+LiCl/HF solutionxe2x86x92LiPF6/HF solution
c) The third step reaction: LiPF6 solution+F2 gas
2. Reaction conditions:
a) The first step reaction: a reaction temperature of xe2x88x9280xc2x0 C. to room temperature, a reaction pressure of 6 to 28 kg/cm2G, and a reaction time of 10 minutes to 3 hours.
b) The second step reaction: a reaction temperature of xe2x88x9230xc2x0 C. to room temperature, a reaction pressure of 1 to 20 , kg/cm2G, and a reaction time of 1 to 3 hours.
c) The third step reaction: a F2 gas flow rate of 1 to 1,000 g/hr, a reaction temperature of xe2x88x9210xc2x0 C. to 0xc2x0 C., and a contact time of 10 to 30 minutes.
3. Purification condition: recrystallization at a temperature of xe2x88x9280 to 0xc2x0 C.
The present invention having a number of characteristics as mentioned above is a method of preparing lithium hexafluoro phosphate (LiPF6) at a high yield of 90% or more, a high purity of 99.8% or more, a low moisture content of 20 ppm or less, and a low free-HF content of 150 ppm or less.
Meanwhile, Japanese Patent Laid-Open Publication No. Heisei 4-175,216 discloses a method in which PCl5 and HF are reacted to synthesize PF5 gas, and a solution of HF and LiF is prepared in a separate vessel, to which the synthesized PF5 gas is then introduced to prepare LiPF6. Such a preparing method is carried out under the following conditions: a molar ratio of HF to LiF ranging 10 to 20, a reaction temperature of xe2x88x9230 to 0xc2x0 C., an introducing flow rate of PF5 of 5 to 30 liter/hr, and a particle size of the produced LiPF6 of 1 to 6 mm. A drawback with this method is that an expensive LiF is used as a raw material. Another drawback with the method is that lithium hexafluoro phosphate (LiPF6) is obtained in a yield of below 70%. The reason for the latter drawback is because PF5 gas is similar in boiling point with HCl, which is produced along with PF5 from the reaction of PCl5 and HF, and is lost to a great extent during a removal of HCl for the reduction of pressure reaction.
The Japanese Patent Publication mentioned above says that the moisture content in LiPF6, a final product, is no more than 10 ppm, but there is no mention indicating that a dried HF is used as a raw material. Thus, it is believed that a product prepared according to the method of the Japanese Patent Publication clearly contains an impurity of LiPOXFY. It also appears that, in the method of Japanese Patent Publication, a vacuum-drying would have to be conducted for an extended period of time in order for a final product to contain moisture of 10 ppm or less. Meanwhile, Japanese Patent Laid-Open Publication No. Sho 60-251109 discloses a method in which PCl5 reacts with HF+LiX(X=Cl, F, I, Br), etc., in a single reactor. In this reaction, the ratio of HF to LiX is 20 to 50 wt/wt.
As previously described, this method has a drawback in that metal components inherent in PCl5, LiX, and the like may be contained in LiPF6, a final product, due to the use of the single reactor. Further, there is another drawback in inevitably requiring several repetitions of recrystallization for removing the metal components, and it is difficult to use the product as a battery cell-grade materials because its yield is no more than 82% and its purity is only 99%.
Furthermore, the document makes no mention of moisture contents and metal components, which would be counter evidence of low purity.
In Japanese Patent Laid-Open Publication No. Heisei 5-279003 the patent disclosed a method wherein PCl5 reacts with HF gas to produce PF5 and HCL gas, the produced PF5/HCL gas passes through a cooling tower at a temperature of xe2x88x9240 to xe2x88x9280xc2x0 C., and the passed PF5/HCL gas is introduced into a reactor containing LiF dissolved in HF to produce LiPF6.
A drawback with this method is that a yield of the reaction is no more than 65%.
EP 0643433 A1 refers to a method for directly producing LiPF6 (EC/DEC) by dissolving the starting raw NH4PF6 in a battery cell-grade solvent, such as EC, DEC or the like and reacting the resultant solution with LiH. This reaction produces NH3 and H2 gases, and the former must be necessarily removed because it remains dissolved in the solvent.
In this process, the amount of used EC or EC solvent decreases as the reaction progress; thus the solvent must be compensated to conform with a desired concentration. Also, the reaction is only valid on the assumption that the conversion is 100%. However, it still seems that it cannot be used as electrolyte solution because there is great probability of existence of unreacted material (NH4PF6, LiH) and because there is no process for removing metal components associated with NH4PF6 and LiH, etc. Comparison between the present invention and the prior art as mentioned above is summarized in Table 1 below.